There are various control methods for direct current to alternating current (DC-AC) converters. In these methods, there are three kinds of controlled current, such as filter inductor current, filter capacitor current, and load current. According to some researches about the control methods, a system controlled by the filter capacitor current has better performance as a whole. Not only cheaper detection devices or circuitry can be utilized to the system, but the dynamic response of the system is better. However, the filter capacitor current itself has ripples. Because of the ripples, accuracy of measurement shall be concerned.
In general, there are few measurement methods for the filter capacitor current, such as direct detection and indirect detection. In the direct detection of filter capacitor current, a hardware detection circuit is utilized to directly measure the filter capacitor current. Values of the filter capacitor current are determined by impedance values of filter capacitors. Since the impedance values of the filter capacitors are usually small, values of the filter capacitor current are also small. Thus, cheaper detection devices can be used in the direct detection. However, the filter capacitor current itself has ripples. If accurate signals are needed, a filter circuit shall be equipped additionally. However, the filter circuit may bring out a signal distortion and delay problems.
There are two methods in the indirect detection. One method is detecting the filter inductor current and the load current by using hardware circuits in advance. The filter capacitor current is the difference value between the filter inductor current and the load current. This method requires at least two detection devices having a wide bandwidth. Therefore, costs of the detection devices or the circuitry are still high and there still exists the delay problems.